Off-line filtration, whether permanently installed on a hydraulic system, or effected by the temporary installation of a portable filter cart, is a powerful bazooka in the contamination control effort.

Without interfering with the operation of the equipment, the oil can be kept clean, in the former case, with full-time, offline filtration. Or, the oil can be flushed periodically with a portable rig, in the latter.

When using a filter cart, the oil is normally taken from the hydraulic tank, filtered and returned to the hydraulic tank. Which means the cleanliness of the newly filtered oil is diluted by the bulk volume of unfiltered oil remaining in the tank. To overcome this dilution effect, as a rule of thumb, the tank volume should be passed through the filter seven times to achieve the equivalent of single-pass filtration (where the oil is pumped through the filter from one container into another).

For example, if you have a 50-gallon tank and a filter cart that pumps at 5 GPM, then according to the rule of seven, you need to run the filter cart for 70 minutes to achieve the equivalent of single-pass filtration (50 gallons multiplied by 7; divided by 5 GPM = 70 minutes). If you want to achieve the equivalent of two-pass filtration, then the offline filter must be engaged for 140 minutes.

The rule of seven is normally used as a guide where portable, particle-counting equipment is not available. In practice, if portable, particle-counting equipment is available, the oil in the hydraulic tank is filtered for as long as it takes to achieve the required cleanliness level.

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